A basic structure of a progressive-power lens is first described with reference to FIG. 8. The progressive-power lens comprises a distance portion having refracting power (power) for distant view, a near portion having power for near view, and an intermediate portion which is disposed between the distance and near portions and progressively varies its power. The power difference between the distance portion and the near portion is known as addition power, which is set at an appropriate value according to the level of accommodation of a wearer of glasses. There are also a pair of aberration portions, which are not suitable for optical use, with the intermediate and the near portions on their sides interposed between the aberration regions. These aberration regions produced by smoothly connecting the power difference between the distance and near portions inevitably exist in a progressive-power lens as its drawback. A spectacle lens also consists of an object-side refractive surface and an eyeball-side refractive surface as illustrated in FIG. 9.
A typical progressive-power lens has measuring points at which respective power for distant and near views are measured. These points are specified by a lens manufacturer and are in general clearly shown on a refractive surface of a lens by printing or other method as illustrated in FIG. 10. When these points are not printed, the measuring points or other signs can be detected from permanent marks attached to the lens in accordance with the specifications of the manufacturer. The points for measuring the power for distant view and near view are referred to as a distance reference point and a near reference point, respectively.
As illustrated in FIG. 11, the refractive surface, which is known as a progressive surface and causes addition power peculiar to progressive power, often lies on the object-side refractive surface. In this case, the eyeball-side refractive surface is formed from a spherical surface or a toric surface with an orientation corresponding to that of a cylinder axis in accordance with a prescribed dioptric power of the wearer of the glasses. This type of the progressive-power lens is referred to as an outside surface progressive-power lens in this description. The outside surface progressive-power lens has a refractive surface for varying image magnification at the object side, which enlarges image distortion. Thus, some people who use the progressive-power lens for the first time or who have replaced a differently designed progressive-power lens with the outside surface progressive-power lens may have a sense of incongruity.
In order to prevent image distortion caused by variations in image magnification, a lens known as an inside surface progressive-power lens has been recently commercialized in which the progressive surface is disposed at the eyeball side as disclosed in WO97/19382 (FIGS. 4, 10 and 15). As illustrated in FIG. 12, the inside surface progressive-power lens has a complicated curved-surface configuration in which the object-side refractive surface is formed from a spherical surface or an aspherical surface symmetric with respect to a rotational axis and the eyeball-side refractive surface is from a combination of progressive surface, a toric surface and a corrective aspherical element for correcting off-axis aberration of the lens.
As described in WO97/19383 (FIG. 1), there has been developed and commercially manufactured another type of lens known as a both-surface progressive-power lens which is formed by dividing addition power element of its progressive surface between an object-side refractive surface and an eyeball-side refractive surface. Since the object-side refractive surface includes a part of the progressive surface element for varying magnification, the both-surface progressive-power lens causes larger distortion than the inside surface progressive-power lens having the object-side refractive surface formed from a spherical surface. Of course, the both-surface progressive-power lens causes less distortion than the outside surface progressive-power lens.
As illustrated in FIG. 12, the inside surface progressive-power lens gains addition power by giving curvature difference between the distance portion and the near portion of the eyeball-side refractive surface. The curvature of the near portion is smaller than that of the distance portion such that the near portion power is greater than the distance portion power. The surface whose curvature is zero has an infinite radius of curvature (=1/curvature), i.e., a flat shape. When the curvature is negative, there fractive surface at the eyeball side is convex toward the eyeball. This is undesirable in view of manufacture, since a curved surface having a concave at the distance portion and a convex at the near portion requires a more complicated processing technique than the curved surface having concaves at both distance and near portions. Thus, the curvature at the near portion needs to be established within a range in which it does not become negative. In this description, the center of the radius of curvature lies closer to the eyeball than to the curved surface when the curvature is positive, and the center of the radius of curvature lies closer to the object than to the curved surface when the curvature is negative, though no distinction between positive and negative is conventionally made for a curvature.
The minimum curvature at the near portion of the eyeball-side refractive surface has been determined because of its processing limitation. For example, a surface having a radius of curvature larger than 1.5 m cannot be processed due to the restriction of a processing machine. Thus, the curvature at the near portion of the eyeball-side refractive surface (=1/radius of curvature) is approximately 0.67 m−1. When a refractive index n of a lens base material is 1.67, a surface power at this curvature can be calculated by a formula of (n−1) X (curvature), obtaining a value of approximately 0.45−1. The power is conventionally shown in dioptre (D=m−1), which unit will be used hereinafter in view of convenience for the calculation of power. The above surface power is thus expressed as 0.45 D.
The surface power at the distance portion must be larger than that at the near portion by the amount equivalent to the addition power which is to be gained by the inside curved surface. For obtaining addition power of 3.00 D, for example, the distance surface power needs to be 3.45 D when the near surface power is 0.45 D. For obtaining the distance portion power of +3.00 D for this lens, the surface power of the object-side refractive surface needs to be 6.45 D as the surface power at the distance portion of the eyeball-side refractive surface is 3.45 D. In general, the required surface power of the object-side refractive surface is slightly smaller than the above value, considering the effect of central thickness of the lens. However, the effect of central thickness is not taken into account herein for simplifying the explanation. The value of 6.45 D as the surface power corresponds to the radius of curvature 0.10 m=100 mm for the lens having a refractive index of 1.67. When the aperture of the lens is 70 mm, the height of the convex is approximately 6.3 mm.
Turning to the outside surface progressive-power lens having a progressive surface at the object side, the distance portion of the object-side refractive surface needs to have a surface power of 3.45 D so as to gain a distance portion power of +3.00 D if the eyeball-side refractive surface having a minimum surface power of 0.45 D is manufactured. The near portion is required to have a surface power of 6.45 D to obtain an addition power of 3.00 D. Thus, the radius of curvature of the object-side refractive surface reaches 0.19 m corresponding to the power of 3.45 D at the upper portion of the lens, and the radius of curvature gradually decreases toward the lower portion of the lens to reach 0.10 m corresponding to the power of 6.45 D at the near portion. The average value thus lies in the range between 3.45 D and 6.45 D, which makes the height of the convex smaller than that of the inside surface progressive-power lens as described. Conversely, the inside surface progressive-power lens is superior in view of optical characteristics such as less distortion, but is inferior in lens thickness and appearance.
In the both-surface progressive-power lens in which the object-side refractive surface has a part of the addition power, it can be easily understood that this lens possesses intermediate characteristics between the outside and inside surface progressive-power lenses. However, the both-surface progressive-power lens involves a higher manufacturing cost due to its complicated processes and longer processing time since the progressive surface is formed from freely curved surfaces having complicated configurations.